Ceramic printing on glass is well known. U.S. Pat. No. 4,321,778 (Whitehead), U.S. RE 37,186 (Hill), WO 00/46043 (Hill and Clare), WO 98/43832 (Pearson) and U.S. Pat. No. 5,830,529 (Ross) disclose partially printed glass panels with a plurality of superimposed layers, including panels variously described as one-way vision panels, vision control panels or see-through graphics panels, and methods of producing such panels. U.S. RE 37,186 describes several methods for the partial printing of a transparent substrate with an opaque “silhouette pattern” comprising layers of ink in substantially exact registration, to produce a panel having a design visible from one side but not visible from the other side and, optionally, a black layer facing the other side to maximise “through vision” from the other side. Three of these methods are referred to as the “direct”, “stencil”, and “resist” methods, all of which involve the removal of cured ink to leave the desired “silhouette pattern” in substantially exact registration. This removal of unwanted ink is undertaken by the application of an overall force applied to the superimposed layers of ink (in the case of the direct and stencil methods) or an overall application of solvent in the case of the resist method. GB 2 188 873 (Hill) discloses improvements to these methods of printing with substantially exact registration and discloses the lateral registration of separately printed areas of ink. WO 00/46043 (Hill and Clare) discloses a range of methods of printing such panels with ceramic ink in substantially exact registration, unified by the printing of superimposed layers onto a base layer and the removal of unwanted ink by a selective force.
WO 04/030935 (Hill and Quinn) also discloses the partial printing of glass panels with ceramic ink in a plurality of layers in substantially exact registration. The substantially exact registration is achieved by the printing of superimposed layers of ink, one of the layers comprising ink with a high proportion of glass frit in a “print pattern”. These layers of ink may be applied directly to a sheet of glass or be transferred as a decal onto a sheet of glass. The glass and the applied layers of ink are subjected to a heat treatment which causes the glass frit to fuse to the glass and bind the layers of ink to the glass within the print pattern. The ink not within the print pattern is burnt off in the heat treatment process and/or otherwise removed in a subsequent finishing process, to leave the desired layers of ceramic ink in substantially exact registration within the print pattern. The invention can be used for the manufacture of one-way vision panels and other products in which the substantially exact registration of layers of ink with at least one common boundary on glass is desired. Alternatively, areas of ink with spaced apart boundaries are laterally registered one to the other. This method has been referred to as the “frit-loaded” method as the substantially exact registration of layers is achieved by “excess” glass frit in one ink layer defining the print pattern. A disadvantage of this method is that any exposed layer initially without frit has a relatively matt appearance compared to conventional ceramic ink fused into glass. Also, so-called one-way vision panels featuring a design visible on one side which is desired not to be visible from the other side optionally comprise a single layer of black frit-loaded ink, which typically has a glossy appearance in some areas but has a relatively matt appearance in other areas of the same black ink in which part of the frit has migrated into a design ink layer. This inconsistent appearance causes a “ghost image” of the design to be visible from the other side, which is typically not desired.
Ceramic ink typically comprises glass “frit”, metal oxide pigments and an ink medium, typically of solvent, resin and plasticiser, in which the pigment and frit are suspended. Frit is glass which has been melted and quenched in water or air to form small particles, which are then ground or “milled” to a desired maximum particle size, typically 10 micron. Ceramic ink may contain oil such as pine oil. Ceramic inks can be opaque or translucent. The ink medium is sometimes referred to as just a medium, a binding medium or a matrix.
Solvent in a ceramic ink medium evaporates following printing, in an ink drying or curing process, leaving resin and plasticiser in the interstices between the glass frit and pigment.
Removal of this resin and plasticiser matrix in the firing of ceramic inks is potentially problematical and a “slow-firing” regime is generally considered preferable, although the firing of ink in a relatively short toughening cycle is known in the art.
The glass is optionally toughened, sometimes referred to as tempered, in the heat treatment process, typically as a second stage following a first stage slow heat treatment process or “ink fusing regime” in which the print pattern is fused to the glass.
GB 2 174 383 (Easton and Slavin) discloses methods of decorating glass with ceramic ink by means of waterslide transfer and a single stage toughening and decal fusing process.
Another type of vision control panel is disclosed in EP 0880439, comprising a transparent or translucent sheet and a transparent or translucent “base pattern” of a different colour to the “neutral background” of the sheet.
Known methods of ceramic decal transfer include:    (i) indirect transfers, for example waterslide transfers and indirect heat release transfers, and    (ii) direct transfers, for example direct heat release transfers.
A transfer process comprises material to be transferred, commonly referred to as a decal (abbreviation of decalcomania), being transferred from a transfer carrier, commonly referred to as a decal carrier, onto a substrate surface.
An indirect transfer method is one in which the means of release of the decal from the decal carrier and the means of adhering the decal to the substrate are typically combined in a single layer on the transfer carrier. The decal is first removed from the carrier and then positioned on the substrate by means of a pad, roller, by hand or other intermediate surface.
For example, a ceramic ink waterslide transfer typically comprises a mass produced decal carrier, typically a specially prepared paper with a sealant layer and a water-soluble adhesive layer. This is optionally printed or otherwise coated with a downcoat, typically a methyl methacrylate based lacquer. It is then printed with the desired layers of ceramic ink forming the required image and then a covercoat is applied, typically a butyl or methyl methacrylate based lacquer. This transfer assembly is typically soaked in water and the decal comprising the covercoat, ceramic ink, optional downcoat and some adhering water-soluble adhesive is released from the carrier and then applied to the substrate surface to be decorated, typically by hand.
As another example, an indirect ceramic ink heat release transfer typically comprises a mass-produced decal carrier, comprising a paper, a sealant layer, a combined heat-activated release and adhesive layer, typically a modified wax incorporating an adhesive or tackifier blend. This is optionally printed or otherwise coated with a downcoat, typically a methyl methacrylate lacquer. It is then printed with the desired layers of ceramic ink and then a covercoat is applied, typically a butyl or methyl methacrylate based lacquer. The decal is then released by applying heat, typically by a heated steel plate under the paper, which activates the release/adhesive layer and allows the decal to be removed from the carrier and then be transferred to and adhered to the substrate to be decorated via an intermediate pad, roller or by hand.
A direct transfer method is one in which a transfer assembly is applied directly to a substrate and the decal carrier is released and removed, leaving the decal on the substrate.
For example, a direct ceramic ink heat release transfer typically comprises a mass-produced decal carrier comprising paper, a sealant layer and a heat release layer, typically a polyethylene glycol (PEG) wax. This is optionally printed with a covercoat, typically a film-forming covercoat, for example of butyl or methyl methacrylate. It is then printed with the desired layers of ceramic ink. Any design is printed in reverse to its intended orientation from the ink side of the substrate. Then a heat-activated adhesive layer is applied, for example a methacrylate resin. This transfer assembly is then typically positioned directly against the substrate with the adhesive layer against the substrate surface. Heat is applied via the paper, which simultaneously activates the adhesive layer and the separate heat release agent. This enables the decal of adhesive, ceramic ink and any covercoat to be adhered to the substrate and be transferred from the carrier, the carrier being released and removed from the decal and substrate. The substrate may optionally be pre-heated.
The terms “covercoat” and “downcoat” are always used in relation to their position with respect to the substrate, a covercoat being a layer over the ink on the substrate and a downcoat being a layer adhered to the substrate, underneath the ink on the substrate.
Typical substrates onto which ceramic decals are transferred include ceramic holloware, ceramic flatware, hollow glassware and flat glass.
All of the above transfer materials and methods are well known in the art.
Many automatic methods of decal application have been devised, for example all the mechanical processes, firing ovens and furnaces described in WO 98/43832.
After ceramic ink is applied to a normal sheet of flat glass, sometimes referred to as float glass and sometimes referred to as annealed glass, the printed sheet of glass is then typically subjected to a thermal regime of up to a temperature of typically 570° C., which burns off all components of the ceramic ink other than glass frit and pigment and melts the glass frit and fuses the remainder of the ink onto the glass, typically followed by relatively slow cooling to anneal the glass once again, which process will be referred to as an “ink fusing regime”. Optionally, annealed glass substrates with ceramic ink can undergo a tempering or toughening regime, which involves raising the glass temperature to typically between 670° C. and 700° C., in which temperature range the glass is relatively soft, and then cooling it relatively quickly, typically by cold air quenching. This causes differential cooling of the glass sheet, the two principal surfaces solidifying before the core solidifies. The subsequent cooling and shrinkage of the core causes a zone of precompression adjacent to each principal surface. The physical strength properties of the glass sheet are fundamentally changed by this glass tempering or toughening regime, which imparts a considerably improved flexural strength to the resultant tempered or toughened glass. Such a glass tempering or toughening regime may be carried out after a separate ink fusing regime or as one process, the ink being fused onto the glass as part of that one process.
With either the ink fusing regime or the glass tempering regime, any transfer process adhesive, covercoat, downcoat and ceramic ink medium are burnt off in the furnace and do not form part of the resultant panel.
It is known in the art to print a design using ceramic ink with a relatively low proportion of glass fit, to intensify the perceived colours, and then overprint with an overall layer of clear transparent ceramic ink with glass frit, sometimes referred to as flux, to “bind in” the pigments below. U.S. Pat. No. 3,898,362 (Blanco) discloses a method of producing an overglaze ceramic decal by wet printing a design layer, free of glass, on a backing sheet and separately depositing a protective coating of pre-fused glass flux on the wet design layer. U.S. Pat. No. 5,132,165 (Blanco) and U.S. Pat. No. 5,665,472 (Tanaka) disclose improvements to this process. Blanco also discloses the prior art lithographic decal method of printing a layer of the desired pattern for one pigment in a clear varnish and then dusting the pigment of the entire sheet in a lithographic process, cleaning the sheet and leaving the pigment only where the varnish is. If more than one colour is required, the process must be repeated and dried between each stage.
EP 1 207 050 A2 (Geddes et al) discloses a transfer system in which a digitally printed ceramic colorant image is applied to a backing sheet followed by an overall overcoat containing frit and binder. Geddes also discloses the thermal transfer digital printing of inks without frit.